AP-firing fast-twitch fibers display a biphasic G_m as a result of alterations in G_Cl and G_K. Experiments similar to that in Fig. 2 were conducted in muscle fibers incubated in solutions with reduced or no extracellular Cl⁻. (A–C) Recordings from a control fiber at 127 mM Cl⁻ (A) that was included for comparison with the representative fiber at 50 mM Cl⁻ (B) and the fiber in the Cl⁻-free solution (C). Note that in Cl⁻-free solutions, 10 nM TTX had to be included in the extracellular solution to prevent myotonic discharges when the electrodes were inserted. Enlargements of the underlined parts in the recordings of the membrane potential have been included below. (D) The mean G_m in fibers in the control solution of 127 mM Cl⁻ (n = 18) and in fibers at 50 mM Cl⁻ (n = 9). (E) The mean G_m in Cl⁻-free solution (n = 11) and the composite conductances, G_K and G_Cl, calculated from recordings in fibers at 127 and 50 mM Cl⁻ using Eq. 4. Mean data are presented with every fifth error bar (SEM).